The present disclosure relates to bolted attachments for attaching wind turbine parts.
It is known in the field of wind turbine applications that bolted attachments for attaching parts, such as for example the blades to the hub, usually do not last the whole wind turbine generator life. As a preventive maintenance, a specific number of bolts is usually required to be replaced over the service life of the wind turbine parts. Frequent replacement of bolts results in increased costs resulting in that the wind turbine is less attractive for the customer.
In the particular case of wind turbine blade bolted attachment, efforts to minimize the above mentioned drawback have been made in an attempt to reach the required life for the bolted attachment. For example, it has been proposed to provide an extender between the pitch bearing and the blade in order to extend the service life of the attachment. However, this has a number of disadvantages. For example, the extender adds extra costs to the wind turbine and problems arise with the transport of the rotor assembly by truck as maximum height is usually exceeded if the extender is assembled in the assembly plant. On the other hand, if the extender is assembled to the blade in the blade supplier plant, blade costs are increased and logistics becomes difficult in order to prevent delays in planning. Furthermore, if the extender is installed on the site, the time spent in the installation process becomes longer, and the process becomes more expensive and more complicated.
The use of circumferentially spaced blade bolts for attaching wind turbine blades to a wind turbine rotor hub. The blade bolts have opposite cylindrical threaded end sections for physical engagement with said parts of the wind turbine, and a non-cylindrical shank extending between aid end sections. One or more flattened sides are defined in the shank along a longitudinal axis of the bolt.
The bolts may have advantageous load and fatigue characteristics to improve fatigue behaviour without reducing the extreme load strength of the bolts. However, bolts are not axisymmetric since their shanks are non-cylindrical due to the flattened sides defined along the longitudinal axis of the bolt. This results in the disadvantages that the assembly of the bolts has to be carried under rigorous control, and that manufacturing of bolts is difficult and highly increases bolt costs.
Therefore, the need for bolted attachments for attaching wind turbine parts still remains such that improved fatigue characteristics are provided without reducing the extreme load strength of the bolts and while avoiding added costs and complexity to the assembly.